In Firefox, you can bypass SSL security checks in order to do local development on machines you trust the identity of. To do this:
- Type about:config into the URL field of Firefox and hit the enter key.
- Find security.tls.insecure_fallback_hosts, double click its value and type your domain(s) there. For example: localhost,dev.local.
This should resolve any Secure Connection Failed errors if you are seeing these on your development machines, but also, its not something you should do for any website out there, because it can easily compromise security.